The present invention relates to an improvement of the electromagnetic clutch disclosed in the aforesaid copending application and in particular to the construction of such a clutch having lighter and less expensive components. The clutch of this invention may be substituted for a commonly used fan clutch used in large trucks in the United States which is constructed with heavy metal components with the first component being driven by the pulley and having a frictional face which is clutched to or separated from a second frictional clutch face on the driven metal portion carrying the fan blades. When the clutch faces are engaged they transmit full power without a slippage between the frictional faces. The clutch faces are typically engaged by a pneumatic or electromagnetic actuator with the pneumatic actuator requiring the use of air on the vehicle. Typically, air is available on vehicles such as large trucks but is not readily available on construction or farm equipment which either employ fans and could employ a fan clutch.
The typical ON/OFF conventional fan clutch used for large trucks or the like employs metal machine parts which are relatively heavy and take considerable horsepower to accelerate and to overcome the inertia thereof. For example, when the fan is of a 32" diameter, the horsepower used to drive the fan can be as much as 40 to 80 hp for large trucks which have engines in the range of 200 to 600 hp. The machining of the fan clutch metal pieces also adds considerably to the cost of this fan clutch. The ON/OFF fan clutches also must be relatively strong and made of metal to withstand the torque impulse forces which are applied relatively quickly, e.g., in about 1.2 seconds, when the fan clutch goes from fully off, light torque load to a very high, full torque load to accelerate the fan-carrying part to its maximum speed, which is generally about the engine speed which can be 2400 rpm. Thus, it is necessary to build a fairly substantial metal fan clutch to withstand the shock of the sudden inertia load of the stationary driven clutch base and its attached fan. Consequently, current ON/OFF fan clutches are quite large and are substantially made to withstand such shock loads and high forces when the fan blades are being accelerated to rotate at high speeds and subjected to large centrifugal force loads at high rotational speeds. The metal pulley and fan carrying pieces require considerable machining to provide the desired surfaces, reference locations, and bearing seats. Such machining adds considerably to the cost of this clutch.
The ON/OFF fan clutches use frictional faces which wear and generally need to be replaced after 150,000 miles or so in order to meet the warranty requirements, which warrant the fan clutch for about 250,000 miles. Of course, the replacement of the friction faces results in a substantial amount of down time during their removal and their replacement. In contrast thereto, the electromagnetic fan clutch disclosed in the aforesaid patent application uses only rotational parts rather than frictional parts and uses only a magnetic field to transfer the torque of the rotating input pulley part to the driven rotational part carrying the fan blades. With current ON/OFF fan clutches, the sixty or so horsepower used by the onset of the fan load and the considerable horsepower used to accelerate the fan under an impulse load, for example, of 60 hp to turn a large 32" fan, is noticeable for large trucks climbing a grade. When the fan clutch comes on and takes 60 hp, the engine must slow down and often the driver must shift down two gears to keep the vehicle going. In the electromagnetic fan clutch disclosed in the aforesaid application and in this application, there is a relatively soft engagement which can be controlled so as to avoid the full application of 60 hp over the short impulse time of 1.2 seconds or the like, thereby eliminating the need to downshift.
It appears that these current ON/OFF fan clutches may be engaged at least 12% of the time for large trucks or the like. Since the large truck is using the full extent of the fan horsepower with the ON/OFF clutch, there may be as many as 40 to 80 hp being used to drive the fan when there is relatively small need for full engagement, because a partial engagement would satisfy the cooling needs. It is estimated that the electromagnetic fan clutch disclosed in the aforementioned application, because it can supply a modulated torque of less than full torque and more than a minimal torque, would need to be engaged only about 1% of the time. Further, the magnetic clutch may be modulated to increase the torque transfer gradually to eliminate the large shock loads of the ON/OFF fan clutch.
An additional consideration with the ON/OFF fan clutch is that of the large amount of noise generated by the fan which is rotating at high speeds for 12% of the time. It is estimated that as much as 50% of the noise from a large truck emanates from the fan which is beating or slapping the air as it rotates at about engine speed to cool the engine. A reduction in noise level using a modulated fan clutch could open new markets for equipment using fans and fan clutches where the equipment cannot be used in the evenings or at night if it exceeds regulated noise levels.
In Europe and in other places such as South America there is often used a viscous fluid fan clutch which is always rotating at speeds of 400 to 600 RPM, even when the fan is turned off. That is, the viscous fluid rotates the fan because of the friction and shears. When the viscous fan clutch is turned on, it never is able to produce or transfer 100% of the input power or torque because 7-10% of the power is lost with viscous fluid shearing. Because of the fluid viscosity and friction, this fan clutch is never totally disengaged to be free-wheeling as it is constantly engaged so that it is always using a considerable amount of the power. This power, of course, is wasted fuel consumption, which makes is a relatively inefficient fan clutch from a fuel economy standpoint. This particular fan clutch also uses a bi-metallic thermostat on the front of the fan assembly to measure temperature and a plunger is operated by the thermostatic switch, which requires ram air to operate. On slow moving construction vehicles or the like where there is relatively little ram air, such a thermo-statically controlled fan clutch is not readily usable. Likewise, for an ON/OFF fan clutch used on trucks there is a requirement for the use of compressed air which is often not available for construction or farm equipment and therefore makes the clutch less saleable to makers of such equipment.
Currently, it is desired to eliminate the 7-10% viscous shear inefficiency without an increase size of the radiators and to provide this increased efficiency to power the fan in order to run the engines hotter using the same cooling equipment. The magnetic fan disclosed in the aforesaid parent application can be used with these hotter engines because it transfers 100% of the power using a magnetic field and does not have the 7-10% inefficiencies due to the viscosity of the viscous fluid.
In all of these fans and fan clutch systems, removal of heat is a significant problem for the bearings and for the fan belts. The failure of bearings or the breaking of fan belts are a considerable cause of maintenance or down time. By keeping the bearings and fan belt temperature lower, the life of the belts and bearings can be improved considerably. These are important considerations with respect to the maintenance and the overall down time with respect thereto caused by bearing or belt failures. The replacement of a bearing on a fan clutch is a significant maintenance problem for a truck or a piece of equipment. Thus, there is a need for a new and improved magnetic clutch that overcomes the deficiencies of existing ON/OFF frictional face clutches and existing viscous fluid clutches.